Near top dead center on the exhaust stroke, the exhaust valve hasn't finished closing yet and the intake valve is already cracking open. That window — measured in crankshaft degrees — is called valve overlap, and it's one of the most powerful tuning knobs a cam designer has. Get it right and you scavenge spent gases for free; get it wrong and you have a truck that won't idle or a race engine that won't pull off the line.

Why overlap exists: Exhaust gas leaving the cylinder at high velocity creates a low-pressure pulse behind it. If the intake valve opens while that pulse is still active, the departing exhaust literally pulls fresh charge into the cylinder before the piston even starts descending. This is called scavenging, and at the right RPM it can stuff more air into the cylinder than atmospheric pressure alone could push.

The tradeoff: Overlap only scavenges cleanly when exhaust velocity is high — meaning high RPM. At idle and low RPM, exhaust gas is slow and lazy. Instead of scavenging, you get reversion: exhaust pushes backward into the intake, diluting the incoming charge with burnt gas. This is exactly why race cams sound "lopey" at idle — the engine is misfiring on diluted mixture because overlap is too aggressive for that RPM.

• Stock economy cam: 10–25° overlap. Smooth idle, good low-end torque, modest top-end.
• Performance street cam: 40–60° overlap. Some idle roughness, broader powerband.
• Race cam: 70–110°+ overlap. Won't idle below 1,200 RPM, screams at 8,000.

Real-world example: A Honda B18C5 (Integra Type R) runs about 35° of overlap on the high cam lobe via VTEC. Below ~5,800 RPM it uses a mild lobe with minimal overlap for civility. Above the crossover, it switches to the aggressive lobe with much more overlap — which is exactly why the engine transforms character at VTEC engagement. Variable cam timing (VVT) systems do the same thing continuously by phasing the cams, sliding overlap in and out as RPM and load change.

Rule of thumb: Overlap "sweet spot" RPM ≈ overlap degrees × 100. So 50° of overlap peaks scavenging effect around 5,000 RPM. Below that you lose charge to reversion; above that, the intake closes before scavenging finishes. This is rough — exhaust length and intake runner tuning shift it — but it's a useful starting point when picking a cam.

Overlap is also why turbo engines historically ran less overlap than NA: boost pressure on the intake side would just blow straight through to the exhaust, wasting fuel and frying turbos. Modern direct-injection turbo engines exploit this with scavenge mode — deliberately running heavy overlap during spool-up to blast cool air through the cylinder, spinning the turbo faster (Ford EcoBoost calls this technique out specifically).